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Having a BMI of under 18.5 can increase the risk of malnutrition, osteoporosis, anemia, and a range of problems that can result from various nutrient deficiencies. It can also be a sign of a hormonal, digestive, or other problem.

Varying cutoff points

Evidence suggests that the associations between BMI, percentage of body fat, and body fat distribution may differ across populations, due to variations in race and ethnicity.

A Brazilian study, published in 2017, looked at the correlation between BMI and body-fat percentage in 856 adult men and women.

They concluded that to predict obesity-type body-fat percentage:

The standard BMI threshold of 29.9 kg/m2 was appropriate for men.

A more suitable cutoff point for women appeared to be 24.9 kg/m2.

In 2017, Korean researchers pointed out that people in the Asia-Pacific region often have a higher risk of type 2 diabetes and cardiovascular disease at a BMI below the existing WHO cutoff point.

In Korea, they added, there is evidence that almost twice as many people have features of metabolic obesity but a normal weight compared with the U.S.

In 2010, results of a study published in The International Journal of Obesity found that Asian Americans within the healthy weight range were more likely to have symptoms of metabolic syndrome than their non-Hispanic white counterparts.

Doctors may use these variations when treating or advising specific people.

Classifications

BMI (kg/m2)principal cutoff points

BMI (kg/m2)additional cutoff points

Underweight

Severe thinness

Moderate thinness

16.00–16.99

16.00–16.99

Mild thinness

17.00–18.49

17.00–18.49

Normal range

18.50–24.99

18.50–22.9923.00–24.99

Overweight

≥25.00

≥25.00

Pre-obese

25.00–29.99

25.00–27.4927.50–29.99

Obese

≥30.00

≥30.00

Obese class I

30.00–34.99

30.00–32.4932.50–34.99

Obese class II

35.00–39.99

35.00–37.4937.50–39.99

Obese class III

≥40.00

≥40.00

Takeaway

BMI is a useful tool that gives a general idea about whether a person's weight is healthy or not. However, it is a simple tool that does not tell the whole story about people's individual weight and health risks.

Anyone who is concerned about their weight should speak to a doctor, who may also consider the individual's body-fat distribution and the ratio of their waist size to their height. A health professional will also be able to offer advice to suit every individual.

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